1996 Cadillac EL Dorado battery

You only did half of the repair, and the wrong half to start with. The low voltage on the dash gauge means the generator isn't recharging the battery as you drive. That's why the battery ran down and the engine stalled. In the case of GM's poor generator design, they have a huge problem with developing voltage spikes that can destroy the internal voltage regulator and diodes. Those spikes can also interfere with engine sensor signals and cause weird running problems that defy diagnosis.

The battery is the key component in damping and absorbing those voltage spikes. They lose their ability to do that as they age. Had you let a mechanic diagnose the problem he would have replaced the generator, but to reduce the number of repeat failures you need to replace the battery too, which you already did, unless it's less than about two years old. The old battery will work okay, until it fails, in an '86 or older model GM car that has the better generator design.